1. Field of the Invention
This invention relates to an improved method for the preparation of an N-alkylated aspartame derivative, which is a particularly useful sweetening agent.
2. Related Background Art
N-[N-(3,3-dimethylbutyl)-L-xcex1-aspartyl]-L-phenylalanine 1-methyl ester, or neotame, having the formula below, is a highly intense non-nutritive sweetening agent useful to impart sweetness to a wide variety of food products. 
This sweetener, disclosed in U.S. Pat. No. 5,480,668, is approximately 8,000 times as sweet as sucrose, on a weight basis. Thus, very small quantities of this sweetening, agent may be used to produce foods and food products that are equi-sweet tasting to presently available high caloric food products.
Several syntheses of neotame using reductive alkylation of aspartame and 3,3-dimethylbutyraldehyde have been reported. U.S. Pat. No. 5,480,668 discloses a method of adding the 3,3-dimethylbutyraldehyde to a mixture of aspartame and reducing agent in methanol. Sodium cyanoborohydride is disclosed as a useful reducing agent. U.S. Pat. No. 5,510,508 discloses a method using hydrogen at 1 bar or less in the presence of a platinum or palladium catalyst as a reducing agent. In this method, a pH 4.5-5 aqueous-alcoholic solution of aspartame and, 3,3-dimethylbutyraldehyde was treated at room temperature with the reducing agent. The product was purified by precipitation and filtration after evaporation of the alcohol from the solution.
U.S. Pat. No. 5,728,862 describes a method comprising treating a solution of aspartame and 3,3-dimethylbutyraldehyde, in an organic solvent with hydrogen in the presence of a catalyst as a reducing agent. After removal of the catalyst, water was added to form an aqueous/organic solvent solution containing about 17% to about 30% of the organic solvent, by weight, from which the neotame was obtained by precipitation and filtration.
In summary, the preparation of neotame by reductive alkylation of aspartame and 3,3-dimethylbutyraldehyde may proceed by addition of a reducing agent to an aspartame/3,3-dimethylbutyraldehyde mixture or addition of 3,3-dimethylbutyraldehyde to an aspartame/reducing agent mixture, typically in methanol or aqueous methanol. Useful reducing agents include hydrogen in the presence of a palladium or platinum catalyst and hydride reducing agents, especially borohydride reducing agents. However, each of these methods is accompanied by the formation of several impurities, as well as recovery of unreacted starting materials. Since sweetening agents are primarily used in foods for human consumption, it is extremely important that such sweetening agents be produced using methods which provide a high purity product.
Accordingly, it would be desirable to develop very efficient and cost-effective methods of preparing high-purity neotame from readily available or readily obtainable materials.
This invention relates to the synthesis of N-[N-(3,3-dimethylbutyl)-L-xcex1-aspartyl]-L-phenylalanine 1-methyl ester (neotame) via the reduction of a novel imidazolidinone intermediate.
The method of this invention comprises the steps of reducing at least one reactant selected from (i) xcex1-methyl hydrogen-D-2-(2,2-dimethylpropyl)-5-oxo-xcex1-L-(phenylmethyl)-1,4(L)-imidazolidine diacetate (trans imidazolidinone) having the formula: 
(ii) xcex1-methyl hydrogen-L-2-(2,2-dimethylpropyl)-5-oxo-xcex1-L-(phenylmethyl)-1,4(L)-imidazolidine diacetate (cis imidazolidinone) having the formula: 
(iii) an imine having the formula: 
or (iv) mixtures thereof to form N-[N-(3,3-dimethylbutyl)-L-xcex1-aspartyl]-L-phenylalanine 1-methyl ester. In a preferred embodiment of the invention, the imidazolidinone intermediate is formed by the reaction of aspartame and 3,3-dimethylbutyraldehyde. This invention also relates to the novel imidazolidinones used in the method of this invention.
Aspartame I reacts with 3,3-dimethylbutyraldehyde (II) to form neotame (III) under reductive alkylation reaction conditions, as illustrated in Scheme I, below. 
Aspartame (I) and the butyraldehyde (II) reversibly react to form an imine (IV), also called a Schiff""s base, and water. This imine is a condensation reaction product of the aldehyde and aspartame, which may convert back to the starting aldehyde and aspartame on addition of water. Under the reaction conditions specified in the method of this invention, the imine may reversibly cyclize to form a 5-membered imidazolidinone. This imidazolidinone may possess either cis or trans stereochemistry, and undergoes equilibration in solution based on the relative stereochemistry of the 3,3-dimethylbutyryl moiety and the methylene carboxyl moiety at the 2 and 4 positions on the heterocyclic ring. Alternatively, these imidazolidinone isomers may be designated as D- and L-isomers, corresponding to the cis and trans isomers, respectively. Cyclization of the imine provides both cis imidazolidinone (V) and trans imidazolidinone (VI).
According to one embodiment of the method of this invention aspartame (I) may be reacted with 3,3-dimethylbutyraldehyde to form a mixture of imine (IV) and imidazolidinones (V and VI) which are in equilibrium with the starting aspartame (I) and butyraldehyde (II). In the absence of water, the equilibrium balance shifts equilibria formation to the imidazolidinone (V and VI); the presence of water shifts equilibria formation to the hydrolysis products/starting materials aspartame (I) and butyraldehyde (II). Formation of imidazolidinone will occur in both the presence or absence of water. However, use of reaction conditions that favor formation of the imidazolidinone by decreasing the water content of the reaction mixture are preferred. For example, use of drying agents or chemical reagents that react with water irreversibly will shift the reaction toward formation of imidazolidinone.
Advantageously, it has been discovered that the imidazolidinone possesses limited solubility in the solvents and solvent mixtures used in the method of this invention. Removal of imidazolidinone by precipitation of the imidazolidinone from the reaction mixture is another way of shifting equilibrium conditions to favor formation of the imidazolidinone. Solid imidazolidinone may be readily isolated from the reaction mixture by filtration, decantation or centrifugation. The solid material produced by the method of this invention may be composed of a mixture of both cis and trans imidazolidinones. The solid imidazolidinone mixture may be used as isolated from the reaction mixture, or may dried.
As indicated above, appropriate selection of the reaction solvent provides for the isolation of solid imidazolidinone by filtration from the reaction mixture. Generally, formation of the imidazolidinone may be conducted in polar protic or polar aprotic solvents. Solvents that are useful for the formation of imidazolidinone according to this invention include C1-C4 alkyl alcohols, tetrahydrofuran, dimethylformamide, dimethylsulfoxide, ethyl acetate and the like, mixtures thereof or aqueous mixtures thereof. Preferably, imidazolidinone formation may be conducted in polar protic or polar aprotic anhydrous solvents. Exemplary preferred solvents include anhydrous C1-C4 alkyl alcohols, tetrahydrofuran, dimethylformamide, dimethylsulfoxide, ethyl acetate and the like, or mixtures thereof. More preferably, imidazolidinone formation may be conducted in anhydrous C1-C4 alkyl alcohol solvents, and most preferably, imidazolidinone formation may be conducted in anhydrous methanol (absolute methanol).
The pH of the reaction mixture for formation of the imidazolidinone mixture is typically between about 4.0 to about 6.0, more preferably, between about 4.5 and 5.5. If desired, the pH of the reaction mixture may be adjusted accordingly.
A preferred manner of obtaining the starting material(s) used in the method of this invention includes condensation of aspartame and 3,3-dimethylbutyraldehyde to form imidazolidinone. This condensation may be conducted by forming a mixture of the aspartame and aldehyde in a selected solvent or solvent mixture. The aspartame and aldehyde may be added to the solvent or solvent mixture in any order. Imidazolidinone formation may be conducted using a molar ratio of aspartame to 3,3-dimethyl butyraldehyde of about 1:0.8 to about 1:1.1. Preferably, imidazolidinone formation may be conducted by combining using a molar ratio of aspartame to 3,3-dimethyl butyraldehyde of about 1:0.98. Optionally, dehydrating agents, such a trimethyl orthoformate, may also be included in the reaction mixture. If desired, the pH of the reaction mixture may be adjusted accordingly. The resulting reaction mixture, which may be heterogeneous, is heated to a temperature of about 20xc2x0 C. to about 60xc2x0 C., preferably about 35xc2x0 C. to about 45xc2x0 C. for approximately 0.5 to 12 hours, and preferably for 1 to 2 hours. At the end of the reaction, the white solids that have formed are filtered, washed with solvent and dried to provide a solid mixture of cis and trans imidazolidinones (V and VI).
As previously indicated, this invention is directed to a method for the preparation of neotame (III) by treatment of novel imidazolidinone(s) under reducing conditions. According to the method of the invention, the imidazolidinones may be directly converted under reducing conditions to neotame or may undergo ring-opening to re-form imine, which may then be reduced to the neotame. Conversion, under reducing conditions, of each of the cis and trans imidazolidinone isomers forms neotame. Advantageously, conversion of the imidazolidinone mixture, under reducing conditions, forms neotame without generating substantial quantities of di-alkylated impurities. Excess 3,3-dimethyl-butyraldehyde present during conventional reductive alkylations leads to formation of di-alkylated aspartame impurities, e.g. di-alkylated aspartame (alkylated neotame), di-alkylated imidazolidinone or di-alkylated aspartame impurities. Specifically, use of 3,3-dimethylbutyraldehyde forms di-neohexyl aspartame or di-neohexyl imidazolidinone impurities. In the method of this invention, any excess 3,3-dimethyl-butyraldehyde that may lead to formation of di-alkylated impurities may be removed from the imidazolidinone solid mixture by filtration and/or drying of the solid mixture. Accordingly, conversion of the imidazolidinone(s) to neotame may be conducted in the substantial absence of butyraldehyde. However, due to the ability of the imine, imidazolidinones and starting materials, i.e., aspartame and butyraldehyde, to inter-convert in presence of water, some equilibrium level concentration of the 3,3-dimethylbutyraldehyde will be present during the catalytic hydrogenation reaction. Thus, while the formation of di-alkylated impurities cannot be eliminated, they may be substantially reduced using the method of this invention.
Di-alkylated imidazolidinone (VII), di-neohexyl imidazolidinone, may be formed by the reaction of neotame (III) and excess 3,3-dimethylbutyraldehyde, according to Scheme II. Advantageously, the reaction is reversible and the di-neohexyl imidazolidinone hydrolyzes in the presence of water to give neotame and 3,3-dimethylbutyraldehyde, which are readily separable. Thus, this di-alkylated imidazolidinone (VII) impurity may be easily removed. Di-alkylated aspartame, or neohexyl-neotame (VIII), may be prepared by reductive alkylation of neotame with 3,3-dimethylbutyraldehyde and is formed as a by-product impurity of the reductive alkylation of aspartame and butyraldehyde. In contrast to the di-alkylated imidazolidinone, the di-alkylated aspartame does not hydrolyze or decompose in water and thus cannot be as readily separated from the desired neotame. 
Conversion, under reducing conditions, of the imidazolidinone mixture to neotame may be conducted using methods well known to those skilled in the art. For example, reducing conditions, or reducing agents, useful in this invention to convert the imidazolidinone mixture to neotame include catalytic hydrogenation, homogenous catalysis, metal hydride reductions, dissolving metal reductions and other non-metallic reductions, such as reductions effected by hydrazinediimide, silanes, formic acid, photoreductions or enzymatic or microbial reductions, as described by Michael B. Smith, xe2x80x9cOrganic Synthesis,xe2x80x9d McGraw-Hill, Inc., New York, N.Y., 1994, the disclosure of which is incorporated by reference herein. Preferably, the conversion, or reductive alkylation, is effected using, for example, catalytic hydrogenation, zinc and hydrochloric acid, sodium cyanoborohydride, sodium acetoxyborohydride, lithium borohydride, sodium borohydride, iridium triphenyl phosphine, borane in pyridine, or formic acid. Most preferably, the imidazolidinone(s) may be reduced by catalytic hydrogenation.
Conversion, under hydrogenation conditions, of the imidazolidinone mixture to neotame is conducted using a hydrogenation catalyst under a hydrogen atmosphere at a pressure and temperature sufficient to effect the conversion. Any hydrogenation catalyst based on platinum or palladium may be used to catalyze the hydrogenation of the imidazolidinone(s) to neotame. Useful hydrogenation catalysts include palladium on activated carbon, platinum on activated carbon, platinum black or palladium black. Other hydrogenation catalysts include, without limitation, nickel on silica, nickel on silica and alumina, Raney nickel, ruthenium black, ruthenium on carbon, palladium oxide, palladium hydroxide on carbon, rhodium black, rhodium on carbon, and rhodium on alumina. Preferably, catalysts based on palladium or platinum are used.
More preferably, catalytic conversion under hydrogenation conditions is conducted using a 5% palladium on carbon catalyst. The catalyst is present in the reaction mixture in an amount effective to produce neotame in acceptable yield from the imidazolidinone(s). Generally, the weight ratio of catalyst to imidazolidinone(s) is about 0.005:1 to about 0.2:1, and most preferably about 0.05:1. The pressure of the hydrogen gas used to effect conversion under hydrogenation conditions of the imidazolidinone(s) may be from atmospheric to about 1000 psig, preferably, from about 15 psig to about 100 psig. More preferably, the reaction is conducted under a hydrogen atmosphere of about 40 psig to about 60 psig.
The catalytic hydrogenation of the imidazolidinone(s) may be conducted in polar protic or polar aprotic solvents. Exemplary solvents include C1-C4 alkyl alcohols, tetrahydrofuran, dimethylformamide, dimethylsulfoxide, ethyl acetate and the like, mixtures thereof or aqueous mixtures thereof. Preferably, the hydrogenation may be conducted in anhydrous polar protic or polar aprotic solvents. Exemplary preferred anhydrous solvents include anhydrous C1-C4 alkyl alcohols, tetrahydrofuran, dimethylformamide, dimethylsulfoxide, ethyl acetate and the like, or mixtures thereof. More preferably, hydrogenation may be conducted in anhydrous C1-C4 alkyl alcohol solvents, and most preferably, hydrogenation may be conducted in anhydrous methanol (absolute methanol).
The catalytic hydrogenation may be conducted by first mixing the imidazolidinone(s) and hydrogenation catalyst in a selected solvent or solvent mixture. The resulting mixture may then be placed under a hydrogen atmosphere of about 15 to about 100 psig and the hydrogenation may be conducted at a temperature of about 20xc2x0 C. to about 60xc2x0 C., preferably about 35xc2x0 C. to about 40xc2x0 C. for approximately 6 to 48 hours, and preferably for 12 to 16 hours. The resulting solution containing N-[N-(3,3-dimethylbutyl)-L-xcex1-aspartyl]-L-phenylalanine 1-methyl ester can then be filtered to remove the insoluble hydrogenation catalyst. Standard filtration techniques may be used. Preferably, the solution is filtered through a layer of filtering aid, such cellulose or Celite(copyright). After filtration, the volume of solvent or solvent mixture used in the hydrogenation reaction may be reduced by evaporation under reduced pressure followed by crystallization. Advantageously, the recovered hydrogenation catalyst (such as Pd, Pt) may be re-used several times in the reductive alkylation. If other reducing conditions or reducing agents, as described above, are used, the reaction mixture should be treated in a manner consistent with the removal of excess reducing agent and the recovery of the neotame product from the reaction mixture, using conventional techniques.
Purification of the N-[N-(3,3-dimethylbutyl)-L-xcex1-aspartyl]-L-phenylalanine 1-methyl ester may be accomplished by recrystallization or column chromatography. Preferably, the N-[N-(3,3-dimethylbutyl)-L-xcex1-aspartyl-L-phenylalanine 1-methyl ester is purified using the techniques and procedures described in U.S. Pat. Nos. 5,510,508, 5,480,668 5,782,862 and in co-pending application, U.S. patent application Ser. No. 60/110,011, filed Nov. 25, 1998, the disclosures of each of which are incorporated by reference herein.
The preparation of neotame by reductive alkylation of aspartame and 3,3-dimethylbutyraldehyde may proceed without the isolation of imidazolidinone by addition of a reducing agent to an aspartame/3,3-dimethyl-butyraldehyde mixture or addition of 3,3-dimethyl-butyraldehyde to an aspartame/reducing agent mixture at a temperature of about 20xc2x0 C. to about 60xc2x0 C., preferably about 35xc2x0 C. to about 40xc2x0 C. for approximately 6 to 48 hours, preferably for 12 to 16 hours, wherein the reducing agent may be hydrogen in the presence of a palladium of platinum catalyst and the weight ratio of catalyst to aspartame may be about 0.005:1 to about 0.2:1, preferably about 0.05:1, the pressure of the hydrogen gas may be from atmospheric to about 1000 psig, preferably about 15 psig to about 100 psig, and more preferably, about 40 psig to about 60 psig. The reactants may be added in any order.